1. Field of the Invention
The invention relates to a negative-type planographic printing plate precursor capable of direct writing by infrared laser beam and high speed plate-making, and a pile made by directly piling the planographic printing plate precursors without interposing laminated paper.
2. Description of the Related Art
PS plate precursors having an oleophilic photosensitive resin layer formed on a hydrophilic support have been widely used as planographic printing plate precursors. PS plates have been produced commonly by subjecting the recording layer to mask exposure (surface exposure) through a lith film and by dissolving and thus removing the non-image regions thereof.
In recent years, digital technology, by which image information is processed, stored, and outputted electronically by computer, is becoming increasingly popular. Accordingly, various newer image-output methods compatible with digital technology have been commercialized. As a result, there is an urgent need for a “computer to plate (CTP) technology” that allows direct production of printing plates by scanning a high-directivity light such as a laser beam according to digitalized image information without using a lith film, and thus there is also a need for planographic printing plate precursors that are compatible with the CTP technology.
As a planographic printing plate precursor compatible with such scanning exposure, a planographic printing plate precursor in which an oleophilic photosensitive resin layer (hereinafter, referred to as a recording layer) containing a photosensitive compound that can generate an active species such as a radical or Bronsted acid by laser exposure is formed on a hydrophilic support has been proposed and already commercialized. It is possible to obtain negative-type planographic printing plates by scanning the planographic printing plate precursor with a laser according to digital information, generating an active species and thus causing physical and chemical changes in the recording layer, insolubilizing the exposed regions, and developing the images thereon.
In particular, a planographic printing plate precursor in which a photopolymerizable recording layer containing a photopolymerization initiator superior in sensitization speed, an addition-polymerizable ethylenic unsaturated compound, and a binder polymer soluble in alkaline developing solution, and additionally an oxygen-blocking protective layer as needed, are formed on a hydrophilic support (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-228109), is superior in productivity, easier in developing processing, and superior in resolution and inking property and is therefore a desirable printing plate precursor with superior printing properties.
For further improvement in productivity, i.e., in plate-making speed, a recording material that employs a photopolymerizable composition containing a cyanine dye having a particular structure, an iodonium salt and an addition-polymerizable compound having an ethylenic unsaturated double bond as the recording layer and that does not require heat treatment after image exposure has been proposed (see, for example, Japanese Patent Application Publication (JP-B) No. 7-103171), but the recording material is susceptible to polymerization inhibition by oxygen in the air during the polymerization reaction and thus has problems that the sensitivity is lower and the strength of image portions formed in the image region is insufficient.
To address these problems, a method of forming a protective layer containing a water-soluble polymer on a recording layer and a method of forming a protective layer containing an inorganic layered compound and a water-soluble polymer are known (see, for example, JP-A No. 11-38633). Presence of the protective layer enables prevention of polymerization inhibition, acceleration of the curing reaction of the recording layer, and improvement in the strength of the image region.
On the other hand, reduction of the time needed in an exposure step is important for improving productivity in plate-making of a photopolymerizable planographic printing plate precursor that can be easily developed. Usually, the planographic printing plate precursors are supplied in the exposure step as a pile containing, between the precursors, a laminated paper for preventing adhesion of the plate precursors and scratching due to abrasion between the surfaces of the relatively soft protective layers and the aluminum supports. As a result, the time needed for removing the laminated paper results in inefficiency in the exposure step. To improve efficiency in the exposure step, it is desirable to eliminate the step of removing the laminated paper by using a pile containing no laminated paper between the precursors, and thus, there is a need for a method of improving the adhesion resistance of the planographic printing plate precursors and preventing scratching due to abrasion between a protective layer surface and an aluminum support.
To improve the adhesion resistance of the planographic printing plate precursors, it is effective to provide a protective layer containing the above-described inorganic layered compound and water-soluble polymer; however at present the layer still needs to have higher scratch resistance.
For the reasons above, there is a need for a planographic printing plate precursor which effectively suppresses polymerization inhibition in the recording layer, and even in a pile containing no laminated paper between the precursors, provides improved adhesion resistance between the precursors, suppresses scratches caused by rubbing between the surfaces of the protective layers and the aluminum supports of the precursors, and thereby improves the productivity of the plate-making operation.
Accordingly, there is a need for a planographic printing plate precursor that is writable by laser exposure, and composed of a recording layer having thereon a protective layer, and that can improve the productivity of the plate-making operation.
More specifically, there is a need for a planographic printing plate precursor composed of a recording layer that is writable by laser exposure, and a protective layer provided on the recording layer, wherein the protective layer suppresses polymerization inhibition in the recording layer, provides excellent development removability, suppresses adhesion between the front surface of a protective layer and an adjacent support side surface of piled planographic printing plate precursors, and reduces scratchubg between the front surface of the protective layer and the rear surface of the aluminum support.
There is a further need for a pile of planographic printing plate precursors without interposing laminated paper that prevents adhesion between a protective layer and an adjacent rear surface of a support, and reduces scratches on the protective layer, and thereby improves the productivity of the plate-making operation.